1. Technical Field
The present invention relates to a semiconductor device.
2. Related Art
High breakdown voltage metal oxide semiconductors (MOS) which operate in high speed are in demand for improving the performance of semiconductor devices. Examples of suggested MOS devices with such features include lateral diffused MOS (LDMOS) and offset MOS having offset gates or drains.
Such MOS devices include, for instance, a P-type substrate, deep N-type wells formed therein, and P-type shallow impurity regions formed inside the N-type wells. Field-effect transistors (FET) are formed using these shallow impurity regions. These MOS devices are driven in higher voltages compared to common low-voltage transistors. Hereafter, these MOS devices may also be referred to as high voltage MOS (HVMOS) devices.
Since a relatively high operating voltage (approximately 30V or more) is input into the HVMOS for its operation, deviation of operating voltage may cause a negative effect on other low-voltage elements formed in the same substrate. For instance, JP-A-5-129425 discloses a semiconductor device that reduces malfunctions of the low-voltage elements caused by a noise or a fluctuation of a voltage which is input into the high-voltage elements formed on the same substrate as the low-voltage elements. This semiconductor device includes a well formed in the substrate, another well formed inside that well, and the low-voltage elements formed inside the second well. The first well has an opposite conductivity type as that of the substrate, and the second well has the same conductivity type as that of the substrate.
However, protecting the low-voltage elements from the noise and the fluctuation of a high-voltage does not sufficiently reduce the deviation of a substrate potential, and therefore a reliable operation of semiconductor device is not necessarily achieved. Moreover, the high-voltage operation of the HVMOS causes the breakdowns in element isolation structures. These breakdowns generate deviation of the substrate potential, and may damage not only the low-voltage elements but also other elements formed on the semiconductor substrate.